1. Field of the Invention
The invention relates to an image forming apparatus such as a printer, a copying machine or a facsimile apparatus of an electrophotographic printing method.
2. Related Background Art
An image forming apparatus such as a printer, a copying machine or a facsimile apparatus using an electrophotographic printing method is generally provided with a photosensitive drum as an image bearing member. Around the photosensitive drum, substantially in succession along the rotation direction thereof, there are disposed a charging device for uniformly charging the surface of the photosensitive drum to a predetermined polarity and predetermined potential, an exposing device for exposing the surface of the photosensitive drum after being charged to thereby form an electrostatic latent image, a developing apparatus for developing the electrostatic latent image as a toner image, a transferring device for transferring the toner image onto a transfer material (transfer medium) such as paper, and a fixing device for fixing the toner image on the transfer material and making it into a permanent image.
As the developing methods of the above-mentioned developing apparatus, there are a dual-component developing method using, for example, a developer comprising carrier particles (carrier) which are a magnetic material and toner particles (toner) which are a nonmagnetic material mixed together at a predetermined ratio, and a mono-component developing method using a developer constituted by only a magnetic or nonmagnetic toner.
For example, in the dual-component developing method, a magnetic brush is formed on the surface of a developing sleeve having disposed therein a magnet roller which is magnetic field generating means. The developing sleeve is opposed to the photosensitive drum with a minute gap therebetween to thereby form a developing nip portion. The magnetic brush borne on the surface of the developing sleeve is brought into contact with or proximity to the photosensitive drum and further, an alternating electric field is continuously applied to the developing nip portion to thereby shift the toner on the surface of the developing sleeve onto the photosensitive drum.
At that time, as shown in FIG. 12 of the accompanying drawings it is ordinary to make a photosensitive drum peripheral speed (process speed) higher than a developing sleeve peripheral speed to thereby give a predetermined peripheral speed difference. In FIG. 12, when the photosensitive drum peripheral speed is 150 mm/sec., the developing sleeve peripheral speed is 228 mm/sec., whereby the peripheral speed ratio is 1:1.52. The developing sleeve peripheral speed is thus increased relative to the photosensitive drum peripheral speed to thereby give a peripheral speed difference, whereby the toner amount supplied per unit area on the photosensitive drum can be increased and therefore, it becomes possible to sufficiently charge the electrostatic latent image with the toner, and it becomes possible to stably obtain sufficient image density.
Now, many of image forming apparatuses using the electrophotographic printing method are designed to be capable of selecting an image forming mode in which the processing speed by the fixing device is changed so that images of different image qualities can be obtained depending on the kinds of the transfer materials used or the kinds of images a user desires to obtain. For example, there is an image forming apparatus provided with two or more kinds of image forming modes such as an OHT mode for forming an image of high transparency on transparent film for an overhead projector (hereinafter referred to as the “OHT”), a thick paper mode for forming an image on thick paper inferior in fixing, and a gloss mode for forming an image rich in gloss to emphasize the brightness of colors, besides a normal image forming mode.
In the above-described OHT mode and the gloss mode, design is made such that the fixing process speed is reduced and the time required for the transfer material to pass through the fixing device is lengthened to thereby increase the heating time, thereby enhancing the fusibility of the toner, and enhancing the transparency and glossiness.
When the processing speed in the fixing device is reduced as described above, it is also necessary to reduce the conveying speed of the transfer material and therefore, depending on the type of the image forming apparatus, it is also necessary to reduce the peripheral speed of the photosensitive drum in accordance therewith.
In that case, it has heretofore been usual to reduce also the peripheral speed of the developing sleeve in accordance with the reduction in the peripheral speed of the photosensitive drum, and make the peripheral speed ratio between the two the same as that at the normal image forming mode. This is because heretofore, a construction in which the driving of the photosensitive drum and the developing sleeve is effected by a single drive source has been ordinary.
For example, in the above-described example, as shown in FIG. 12, in the normal mode, the photosensitive drum peripheral speed is 150 mm/sec., the developing sleeve peripheral speed is 228 mm/sec. and the peripheral speed ratio therebetween is 1:1.52, where as at the OHT mode, the photosensitive drum peripheral speed and the peripheral speed of the rotation of the developing sleeve are both reduced by a half, to 75 mm/sec. and 114 mm/sec. to thereby make the peripheral speed ratio the same as that at the normal mode, namely, 1.52.
In Japanese Patent Application Laid-open No. S62-98373 and Japanese Patent Application Laid-open No. H4-324469, there is described an apparatus in which the peripheral speed ratio between the photosensitive drum and the developing sleeve is variably controlled.
However, when at an image forming mode in which the peripheral speed of the photosensitive drum is thus made lower than that at the normal image forming mode, the peripheral speed of the developing sleeve is also made low at the same rate without the peripheral speed ratio being changed, the following problems have arisen.
The toner supplied into the developing apparatus rubs against the carrier in the case of the dual-component developing method, or against the developing sleeve or a layer thickness regulating member in the case of the mono-component developing method, to thereby be given a desired charging amount, and thereafter is used for development. At that time, of course, it is desirable that as the toner, the distribution of the charging amount thereof be sharp and the proportion of the toner having an appropriate charging amount be high.
The charging amount of the toner, however, actually assumes a broad distribution having a certain degree of expanse as shown in FIG. 13 of the accompanying drawings, depending on the state of the carrier or the developing sleeve on the side giving charges, or the state of a charge control agent for the toner or an extraneous additive assisting the charging. The toner used here is a negative toner charged to the minus polarity, and the (−) side of the axis of abscissas of FIG. 13 indicates the distribution of a toner charged to a regular polarity, and the (+) side of the same axis indicates the distribution of a toner charged to the opposite polarity.
In the distribution shown in FIG. 13, the toner having a charging amount in the vicinity of 0 (zero) which is indicated by an area “A”, and a toner (reversed toner) charged to the opposite polarity cause the phenomenon of so-called “fog” that they cannot be completely controlled by a developing bias, but adhere to the white background portion of the transfer material.
It has become apparent by the inventors' studies that if in the distribution of the toner charging amount, in a state in which the distribution of the toner having a charging amount in the vicinity of 0 (zero) or the toner charged to the opposite polarity is large, i.e., a situation in which the fog is liable to occur, at an image forming mode whereat the peripheral speed of the photosensitive drum is made low as at the OHT mode and the thick paper mode, the peripheral speed of the developing sleeve is also made low at the same ratio as the speed reduction ratio of the photosensitive drum, the fog is more aggravated. This can be explained as follows.
The toner amount T supplied per unit area of the developing portion (developing nip) during development is proportional to the peripheral speed ratio Vr between the photosensitive drum and the developing sleeve, and the bearing amount M of the developer on the developing sleeve per unit area.T∝Vr×M  (1)
In the case of a dual-component developer, the toner amount T is the value of expression (1) multiplied by toner density (TD) and thus,T∝Vr×M×TD  (2)
A part of this toner amount supplied to the developing nip portion flies to the photosensitive drum by the action of a developing bias, whereby development is effected.
In the distribution of the toner charging amount shown in FIG. 13, the toner having a normal charging amount (the toner in an area other than the area “A”) is liable to follow the developing bias applied to the developing nip portion and therefore, the time required for light portion potential (in the case of reversal development) to be fully charged is sufficiently shorter than a developable time (the time required for the photosensitive drum to pass the developing nip portion), and even if the peripheral speed of the photosensitive drum is made low to thereby lengthen the developable time the developing amount per unit area is hardly charged. That state is indicated by solid line “a14” in FIG. 14 of the accompanying drawings.
Also, in the case of the toner having a normal charging amount, when the toner has entered the developing nip portion, the time required until such toner receives the action of the developing bias therein and the probability of the toner with which it becomes capable of flying to the photosensitive drum is also sufficiently shorter than the toner staying time on the developing sleeve when viewed from the photosensitive drum. Therefore, even if the peripheral speed of the developing sleeve is made low to thereby lengthen the toner staying time, the developing amount is hardly changed. That state is indicated by solid line “a15” in FIG. 15 of the accompanying drawings. However, when the peripheral speed ratio Vr is too small, even the toner having a normal charging amount does not sufficiently rise in charging rate and therefore, depending on the construction of the developing apparatus used, the developing bias, the kind of the developer, etc., it is necessary to provide an appropriate peripheral speed ratio. In the examples shown in FIGS. 14 and 15, Vr is set to 1.5.
In contrast, the toner having a low charging amount indicated by the area “A” in FIG. 13 is bad in its follow-up property to the developing bias and therefore, even when it is supplied to the developing nip portion, it does not immediately move to the dark portion potential (in the case of reversal development) of the photosensitive drum. In such a case, the developing amount (fog amount) comes to depend on the developable time and the toner staying time on the developing sleeve when viewed from the photosensitive drum, and comes to increase in proportion to these times. That is, the lower the peripheral speeds of the photosensitive drum and the developing sleeve become, the more the developing amount (fog amount) increases. Those states are indicated by dotted lines “b14” and “b15” in FIGS. 14 and 15, respectively.
Thus, fog becomes more aggravated when at an image forming mode such as the OHT mode whereat the peripheral speed of the photosensitive drum is made low, the peripheral speed of the developing sleeve is also made low so as to keep the peripheral speed ratio constant.